Description:Figure 1 illustrates a battery management system 100 in accordance with an embodiment of the present invention. The battery management system 100 ensures operational safety of a battery pack 105 that includes a plurality of cells. As shown in Figure 1, the BMS 100 includes a balancing circuit 107 coupled to the battery pack 105 and adapted to perform load balancing of the cells of the battery pack 105. The balancing circuit 107 also includes a measurement module that senses battery pack parameters such as voltage (V), current (I) and temperature (T) with the help of signal conditioning circuit connected with the measurement module. The measurement module converts these analog parameters in digital form and feeds to microcontroller, which processed this data based on pre-defined algorithm to estimate state of charge of the battery pack, as will be explained in the ensuing description. The balancing circuit 107 is further coupled to a protection circuit 110 that senses relevant parameters of the cells/battery pack to detect any condition of overvoltage, undervoltage and overcurrent and take corrective action, as will be explained in the ensuing description. Further, the protection circuit 110 is coupled to a monitoring and control unit 115 that is adapted to detect high temperatures in the cells. The monitoring and control unit 115 include a microcontroller and a brick layer circuit (not shown), which will be explained in conjunction with Figure 2. The primary function of the brick layer circuit is to estimate a value of state of charge for the battery pack 105 for use in detecting undervoltage condition in the battery pack and initiate corrective action. In addition, the monitoring and control unit 115 also helps in detecting a high temperature condition in the cells and initiates corrective action.

Further, as shown in Figure 1, the monitoring and control unit 115 is powered by an Auxiliary Power Unit 120. In addition, a fault indicator 125 is coupled to the monitoring and control unit 115. In an embodiment of the present invention, the fault indicator includes a number of LEDs to provide a visual indication in case any of the overvoltage, overcurrent, undervoltage or high temperature condition is detected in the battery pack. The battery management system 100 also includes a communication unit 130 for communicating with external devices regarding the faults detected in the battery pack. This helps as giving an additional mode of alert signalling. Finally, the BMS 100 also includes a display unit 135 to display the various parameters, like individual cell voltages, battery pack voltage, current and temperature and SOC in real time.

As already stated, the monitoring and control unit 115 helps in estimation of an instantaneous state of charge of the battery cell, which is useful for detecting any undervoltage condition in the battery cell and initiate corrective action. For efficient corrective action, it is imperative that the estimated SOC is accurate and the present invention envisages to ensure that by employing a unique methodology implemented through a brick layer electrical equivalent circuit including a combination two parallel RC units, as shown in Figure 2.

Electrical equivalent circuit models (EECM) of a battery, as shown in Fig. 2, uses lumped electrical components to characterize the static as well as dynamic behaviours of the cell and is found to be a good candidate for the Kalman estimation. This model captures the important battery dynamics well and also easy to implement with high computational efficiency. A brick layer electrical equivalent circuit is represented in figure 2, where OCV is the open circuit voltage of the cell, parallel R-C pairs represent dynamic polarization behaviour of the cell and series resistance R_0 represents the internal resistance of the cell. From the definition of SOC, since Q remains constant, it is evident that SOC changes only with Q_Reswhich, in turn changes with the amount of current passed through the cell i.e.Q ̇_Res= I(t). Using this, the SOC dynamics of a battery is given by

z ̇(t)= -η/Q I(t) ↔ z(t)=z(0)- η/Q ∫_0^t▒〖I(t)dt〗 (1)

where, z(0) is representing the initial SOC, η is the Coulombic efficiency of a battery expressing loss of energy during charging. Normally, η is unity during discharge and close to unity during charge. In this report, it is considered to be unity i.e. 1 in both the cases. The instantaneous current I(t) is assumed to be positive during charge and negative during discharge.

Now from figure,

V ̇_1 (t)=- 1/(R_1 C_1 ) V_1 (t)+ 1/C_1 I(t) (2)

V ̇_2 (t)=- 1/(R_2 C_2 ) V_2 (t)+ 1/C_2 I(t) (3)

V(t)=OCV(z(t))-V_1 (t)-V_2 (t)-R_0 I(t) (4)

Equations 1 to 3 are the state equations and Equation 4 is the output equation. From these equations it is clear that it has linear state equations and a nonlinear output equation because of non-linear relation between open circuit voltage and SOC. Linearizing the output equation by first order Taylor’s series expansion at each SOC points gives the state-space model of the form:
x ̇(t)=Ax(t)+Bu(t)

y ̇(t)=Cx(t)+Du(t)
where,

States = x(t)=〖[V_1 (t) V_2 (t) z(t)]〗^T

Input = u(t) = I (t) andOutput = y(t) = V(t)
[A]= [■(0@(-1)/(R_1 C_1 )@(-1)/(R_2 C_2 ))][B]= [■(1/(3600*C_q )@(-1)/C_1 @1/C_2 )]
[C ]= [(dV_oc (z_0))/dz -1 -1] D = [R_s]

To identify model parameters at various SOC levels, the present envisages that a pulse relaxation test is to be performed, as shown in figure 3. In this a LCO cell of capacity 2.5 Ah has been pulse discharged in the voltage window of 4.2V to 3V from fully charged state (100% SOC) to fully discharged state (0% SOC) and battery model parameters (OCV,R_0,R_1,C_1, R_2,C_2 ) are evaluated at each SOC level using curve fitting technique as shown in figure 4. Particularly, figure 4 shows modelling accuracy in terms of voltage when compared with experimental data of electrical parameter for cell and battery pack.Further, the present invention envisages to use these equations and Kalman filter algorithm, as per figure 5,the SOC is estimated for single cell in battery pack. The algorithm involves two steps – time update step that involves prediction of the present state of the electric cell using prior information, and a measurement update step wherein the prediction is corrected using current measurement. Additionally, an error covariance is also calculated which provides information about the uncertainty of the estimation.The entire algorithm and the background process results in the equations (1) to (4) as above. After estimating SOC for each cell in the battery pack individually, average of the SOC values is determined.

Figure 5A illustrates a battery management method 500 implemented in the BMS 100 for ensuring operational safety of the battery pack 105. The method 500 includes performing at least one of sensing relevant parameters of the cells of the battery pack for detecting conditions of overvoltage, undervoltage, high temperature and overcurrent in the cells using a protection circuit 110, and detecting high temperature in the cells using a monitoring and control unit 115, at step 505. The methid further includes, at 510, providing an indication of at least one of overvoltage, undervoltage, overcurrent and high temperature in the cells using a fault indicator unit 125. In an embodiment of the present invention, the step of detecting undervoltage in the cells includes the step ofestimating an instantaneous state-of-charge (SOC) for each cell based on an initial SOC, Coulombic efficiency of the cell and instantaneous current using Kalman’s estimation as per equation (1)
z ̇(t)= -η/Q I(t) ↔ z(t)=z(0)- η/Q ∫_0^t▒〖I(t)dt〗 (1)
and performing an undervoltage protection of the cells by the protection circuit, as will be explained in the ensuing description.

Further, it is envisaged that the BMS 100 ensures an undervoltage protection of the battery pack. In particular, Figure 6(a) illustrates a method 600 implemented in the BMS 100 for undervoltage protection. At 605, the method includes reading the voltage of each cell of the battery pack 105 and receiving the estimated instantaneous SOC from the monitoring and control unit 115. At 610, the instantaneous SOC of the battery pack is compared with a threshold value of the SOC. At 615, the battery pack is disconnected from the discharge path if the instantaneous SOC of the battery pack is less than the threshold value of the SOC. However, if at 610, if the it is determined that the instantaneous SOC of the battery pack is more than the threshold value of the SOC, then at 620, the voltage across any of the cells is compared with a threshold value of the voltage and the battery pack is disconnected from the discharge path, at 615, if the voltage across any of the cells is less than the threshold value. In an embodiment of the present invention, the predetermined interval is 50 milliseconds. In another embodiment of the present invention, the threshold value of the SOC and the voltage is 20% and 3V, respectively.

Figure 6(b) illustrates a method 630 for overvoltage protection, implemented in the BMS 100, in accordance with an embodiment of the present invention. For over voltage protection of the battery pack, at 635, the protection circuit 110 measures cell voltages across each cell of the battery pack 105. In the event the measured cell voltage of any of the cell is higher than the threshold voltage value, at 640, the protection circuit 110 initiates disconnection of the battery pack from charging. In particular, the protection circuit 110 directs the microcontroller to disconnect the battery pack from charging. In an embodiment of the present invention, the predetermined interval is 50 milliseconds. In another embodiment, the threshold voltage value is 4V.

Further, Fig. 6(c) illustrates a method 650 for enabling over current or short circuit protection of the battery pack. At 655, the protection circuit 110 measures, at predetermined intervals, the current of the battery pack 105. At 660, the measured current of the batter pack is compared with a threshold value of the current. In the event the measured current is higher than a threshold value of the current, then at 665, the protection circuit 110 initiates disconnection of the battery pack from load or charging, as the case may be. In particular, the protection circuit 110 directs the microcontroller to disconnect the battery pack from load/charging. In an embodiment of the present invention, the threshold current value is 25A and the predetermined interval is 50 milliseconds.

Moreover, a method 670 for enabling thermal protection of the battery pack is illustrated in Fig. 6(d). Particularly, the monitoring and control unit 115 measures temperatures of each of the cells of the battery pack 105 at 675, and compares the same with a threshold temperature value at 680. In the event the measures temperature os higher than the threshold temperature value, the monitoring and control unit 115 initiates disconnection of the battery pack from load/charging at 685. In particular, the monitoring and control unit 115 directs the microcontroller to disconnect the battery pack from charging. In an embodiment of the present invention, the predeterminedinterval is 50 milliseconds. In another embodiment of the present invention, the threshold value of temperature is 65º C.
The present invention also envisages that the BMS 100 balances cells in the battery pack for equal load distribution. To aid this, a method 690, illustrated in Fig. 6(e), is implemented in the balancing circuit 107. At 695, at predetermined intervals, the voltage across each cell of the battery pack is measured by the balancing circuit 107 and the cell with minimum voltage is identified at 700. Further, at 705, a difference of the voltage of the identified cell with that of the remaining cells is calculated to find respective delta voltages pertaining to each of the remaining cells. The said delta voltages are compared with a threshold voltage value. At 710, if any of the delta voltage is greater than a predefined value, the balancing circuit 107 discharges the identified cell until the delta voltage is lower than the predefined value. In an embodiment of the present invention, the predetermined interval is 250 milliseconds. In another embodiment of the present invention, the predefined value is 20mV.

Figures 7 (a) and 7(b) illustrate the circuit diagram of the various modules of the BMS 100. Each cell of a battery pack is connected to a battery monitor ICvia a filtering circuit consisting R-C low pas filter circuit.The resistors used in filtering circuits also severs the purpose of balancing resistor and dissipate excess energy of a cell at the time of balancing operation. The values of R-C filtering can be decided based on maximum balancing current, maximum cell voltage and R-C time constant.

The protection circuit mainly consists of MOSFETs and driving circuitry to operate these MOSFETs as switch. To control the bidirectional flow of current (charging - discharging)two MOSFETs are connected to each other by their respective drain terminal. In this way when only one MOSFET is on the flow of current will be unidirectional (either charging or discharging depending on which MOSFET is turned on) and when both MOSFETs are on current can flow in either direction. So at minimum a pair of MOSFETs are required to control both charging and discharging process.

The monitoring and control unit consists a microcontroller. This microcontroller works as a brain of the BMS. It takes sensed data (V, I, T) from battery monitor IC as an input, process these data based on the predefined algorithm to calculate SOC, execute balancing and protection as and when required.

The role of the auxiliary power circuitry to take power from connected battery pack and supply this to low power devices such as microcontroller, display board, communication circuitry and other auxiliary circuit at reduced voltage level.
, Claims: A battery management system for ensuring operational safety of battery pack having a plurality of cells, the battery management system comprising:
a balancing circuit coupled to the battery pack for balancing loads of the cells thereof;
a protection circuit coupled to the balancing circuit for sensing relevant parameters of the cells to detect overvoltage, undervoltage and overcurrent in the cells;
a monitoring and control unit coupled to the protection unit for detecting high temperature in the cells, the monitoring and control unit comprises a a microcontroller and a brick layer circuit comprising a series combination of two parallel resistance-capacitor (RC) circuits; and
a fault indicator coupled to the monitoring and control unit to provide at an indication if at least one of overvoltage, undervoltage, overcurrent and high temperature in the cells,
wherein the monitoring and control unit estimates an instantaneous state-of-charge (SOC) for each cell based on an initial SOC, Coulombic efficiency of the cell and instantaneous current using Kalman’s estimation as per equation (1)
z ̇(t)= -η/Q I(t) ↔ z(t)=z(0)- η/Q ∫_0^t▒〖I(t)dt〗 (1)
for use in undervoltage protection by the protection circuit.

The battery management system as claimed in claim 1, wherein the protection circuit,at predetermined intervals,measures voltage across each cell of the battery pack, compares the same with a threshold voltage value and disconnects the battery pack from charging if the measured voltage across any of the cells is more than the threshold voltage value.
The battery management system as claimed in claim 2, wherein the threshold voltage value is 4V..
The battery management system as claimed in claim 2, wherein the predetermined interval is 50 milliseconds.
The battery management system as claimed in claim 1, wherein the protection circuit at predetermined intervals, measures current of the battery pack, compares the same with a threshold current value and facilitates disconnection of the battery pack from charging or load if the measured current is more than the threshold current value.
The battery management system as claimed in claim 5, wherein the threshold current value is 25A.
The battery management system as claimed in claim 5, wherein the predetermined interval is 50 milliseconds.
The battery management system as claimed in claim 1, wherein the monitoring and control unit, at predetermined intervals, measures temperature of each cell of the battery pack, compares the same with a threshold temperature value and disconnects the battery pack from charging or load if the measured temperature is more than the threshold temperature value.
The battery management system as claimed in claim 8, wherein the threshold temperature value is 65º C.
The battery management system as claimed in claim 8, wherein the predetermined interval is 50 milliseconds.
The battery management system as claimed in claim 1, wherein the balancing circuit, at predetermined intervals, measures voltage across each cell of the battery pack, identifies the cell with minimum voltage, compares the minimum voltage with the voltages of the remaining cells of the battery pack to find respective delta voltage, and if any of the delta voltage is greater than a predefined value, discharges the identified cell until the delta voltage is lower than the predefined value.
The battery management system as claimed in claim 11, wherein the predefined value is 20mv …..
The battery management system as claimed in claim 5, wherein the predetermined interval is 250 milliseconds.
The battery management system as claimed in claim 1, wherein the protection circuit performs undervoltage protection, at predetermined intervals, by reading the voltage of each of the cells of the battery pack and receiving the instantaneous SOC of the battery pack from the monitoring and control unit to facilitate disconnection of the battery pack on satisfaction of at least one of the conditions of the instantaneous SOC being less than a threshold value of SOC and, the voltage of any of the cells being less than a threshold value of voltage when the instantaneous SOC is more than a threshold value of SOC.
The battery management system as claimed in claim 14, wherein the threshold value of SOC is 20%.
The battery management system as claimed in claim 14, wherein the threshold value of voltage is 3V.
The battery management system as claimed in claim 14, wherein the predetermined interval is 50 milliseconds.

A battery management method for ensuring operational safety of any battery pack having a plurality of cells, the battery management method comprising the steps of:
performing at least one of sensing relevant parameters of the cells for detecting conditions of overvoltage, undervoltage, high temperature and overcurrent in the cells using a protection circuit, and detecting high temperature in the cells using a monitoring and control unit;
providing an indication of at least one of overvoltage, undervoltage, overcurrent and high temperature in the cells using a fault indicator unit,
characterized in that the step of detecting undervoltage in the cells comprises the step ofestimating an instantaneous state-of-charge (SOC) for each cell based on an initial SOC, Coulombic efficiency of the cell and instantaneous current using Kalman’s estimation as per equation (1)
z ̇(t)= -η/Q I(t) ↔ z(t)=z(0)- η/Q ∫_0^t▒〖I(t)dt〗 (1)
and performing an undervoltage protection of the cells by the protection circuit.

A battery management methodas claimed in claim 18 comprising performing load balancing of the cells using a balancing circuit, wherein the step of balancing the loads of the cells, comprises the further steps of:
measuring voltage across each cell of the battery pack;
identifying a minimum cell voltage among all the measured cell voltages;
comparing the minimum cell voltage against remaining measured cell voltages by a comparison circuit at a predetermined intervalto determine respective delta voltage;
discharging the cell corresponding to which the delta voltage is greater than a predefined value until the delta voltage is lower than the predefined value.

The battery management method as claimed in claim 19, wherein the predetermined interval is 250 milliseconds and the predefined voltage is 20mV.

The battery management method as claimed in claim 18, wherein the step of detecting conditions of over voltage in the battery pack, comprises the steps of:
measuring voltage across each cell of the battery pack,
comparing measured cell voltages at a predetermined interval against a threshold voltage value by a comparison circuit; and
activating protection by disconnecting the battery pack from charging upon the measured cell voltage across any of the cells being more than the threshold voltage value.

The battery management method as claimed in claim 21, wherein the threshold voltage value is 4V and the predetermined interval is 50 milliseconds.

The battery management method as claimed in claim 18, wherein the step of detecting high temperature in the battery pack comprises the steps of:
measuring temperature of each cell of the battery pack at predetermined intervals;
comparing measured temperatures against a threshold temperature value by a comparison circuit;
activating protection by disconnecting the battery pack from charging or load upon any of the measured temperatures being more than the threshold temperature value.

The battery management method as claimed in claim 23, wherein the threshold temperature value is 65º C and the predetermined interval is 50 milliseconds.

The battery management method as claimed in claim 18, wherein the step of detecting conditions of over current in the battery pack, comprises the further steps of:
measuring the current of the battery pack;
comparing the measured current at predetermined interval against a threshold current value;
activating protection by disconnecting the battery pack from charging or load upon the measured current being more than the threshold current value.

The battery management method as claimed in claim 25, wherein the threshold current value is 25A and the predetermined interval is 50 milliseconds.

The battery management method as claimed in claim 18, wherein the step of detecting conditions of undervoltage in the battery pack, comprises the further steps of:

measuring the voltage each cell of the battery pack;
determination of the instantaneous SOC of the battery pack; and
disconnecting the battery pack upon satisfaction of at least one of the conditions of the instantaneous SOC being less than a threshold value of SOC and, the voltage of any of the cells being less than a threshold value of voltage when the instantaneous SOC is more than a threshold value of SOC.

The battery management method as claimed in claim 27, wherein the threshold value of SOC is 20% and the threshold value of voltage is 3V.